The invention relates to a damper valve arrangement suitable for hydraulic power steering systems, in which a servomotor of the system is designed as a hydrostatic motor unit or as a hydraulic displacer unit. The servomotor also acts as a steering damper. At least one damper valve arrangement is provided in hydraulic lines between the servomotor and the servovalve, having a first flow path which can be controlled by a first liftable valve element in the manner of a non-return valve, and a second flow path which can be controlled with a damping action by a second valve element.
Modern motor vehicles are generally fitted with a power steering system which, at least in passenger cars, generally operates using auxiliary hydraulic force such that the forces which the driver has to apply when actuating the vehicle steering system are always kept at a sufficiently low level.
If a steering damper arrangement is required in order to avoid or suppress vibrations in the steering system, according to German patent document DE-A 40 29 156 it is possible for a double-acting piston-cylinder unit serving as the servomotor to also act as a steering damper. This can occur if damper valve arrangements are provided in the lines between the piston-cylinder unit and the servovalve which is used to control this unit. Via the lines, the piston-cylinder unit can be connected to a hydraulic pressure source and a relatively unpressurized hydraulic reservoir in a controllable manner.
An arrangement which is similar in principle is described in German patent document DE-A 41 06 310.
It is known from German patent document DE-A 29 27 039 (having a counterpart in Great Britain GB 20 51 714 A) to arrange a non-return valve at each of the cylinder-side ports of the lines which connect the servovalve to the double-acting piston-cylinder unit of the power steering system. The non-return valves open toward the respective piston working chamber and are provided with a passage-restricting bore. The casings of the valves may be formed by a screw which is used to attach the corresponding line to the cylinder.
According to German patent document DE 43 23 179 A1, in order to simplify the damper valve arrangements, the damper valve elements are arranged on a perforated-disc-like valve support part. The holes of the support part (through which hydraulic medium passes) can be controlled by spring-loaded valve plates or small valve plates which are held on the valve support part on the end face through the use of bolts. The valve support part may be clamped (in the manner of a spacer ring or a spacer disc) between a bearing surface of a connection piece which is arranged on the casing of the servovalve or of the servomotor and a mating bearing surface of the connection part, which can be joined to the connection piece, of the hydraulic line in question.
As an alternative, it is also possible to hold the valve support part captively in the connection piece or in the connection part, as illustrated, for example, in German patent document DE 44 23 658 A1.
The object of the present invention is to provide a novel damper valve arrangement which can function optimally even under changing operating conditions.
This and other objects are achieved by means of a damper valve arrangement suitable for a hydraulic power steering system, the servomotor of which is designed as a hydrostatic motor unit or as a hydraulic displacer unit while also acting as a steering damper. At least one damper valve arrangement is provided in hydraulic lines between the servomotor and servovalve, having a first flow path, which can be controlled by a first liftable valve element in the manner of a non-return valve, and a second flow path, which can be controlled with a damping action by a second valve element. The first valve element is designed as a liftable rotary slide valve which is rotationally adjustable as a function of temperature and, in the event of rotational adjustment, closes or opens, in the non-return direction, the first flow path and/or a bypass.
The invention is based on the general concept of compensating for temperature-dependent changes in the viscosity of the hydraulic medium and the associated change in the flow resistances in the non-return direction, by opening a flow path. The flow path bypasses the restrictive second valve element, to an increasing extent at low temperature, when the hydraulic medium becomes comparatively viscous.
In principle, this ensures that the first valve element, firstly due to its lifting mobility, interacts with flow paths which it controls in the manner of a non-return valve, and, by means of an additional rotational mobility, is able to open the above-mentioned flow paths and/or a bypass independently of its lifting position.
According to a preferred embodiment of the invention, the damper valve arrangement has a support part which is in the form of a perforated disc. The bores in the disc form the flow paths. A spring-loaded valve plate interacts with some of the bores in the manner of a non-return valve. Rotation of the valve plate allows one or more openings or cutouts in the plate to be made to overlap, to a greater or lesser extent, with at least one bore which is controlled by the valve plate.
The rotation of the valve plate can be brought about by using bimetallic elements which, if appropriate, may at the same time assume the function of a closure spring which displaces the valve plate towards the support part.